The invention relates to a pressure relief valve for use in an anaerobic digestor tank and more particularly to a relief valve for providing fluid flow through the valve if the differential pressure on opposite sides of the valve exceeds a specified amount.
Anaerobic digestor tanks have been used as an effective and economical method of treating and handling biological sludge resulting from treatment of waste water. In such a process, a waste water influent is mixed with a microorganism-containing recycled biomass to form a mixed liquor. The environment of the mixed liquor is appropriately controlled to cause growth of and maintain a satisfactory population of microorganisms which assimilate and anaerobically metabolize the organic waste in the liquor.
A by-product of the anaerobic process includes methane gas It is desirable that the accumulated gas be released under controlled conditions so that it may be used for other purposes. For example, the methane gas may be burned to generate heat, and this heat can be applied to the tanks in order to increase the temperature of the mixed liquor and the rate of anaerobic digestion. To provide for controlled release of methane gas from the digestor, a pressure control valve is provided. Further, a relief valve is provided to relieve the pressure beyond a set limit if the gas production exceeds the rate of use. This relief valve should also permit air flow into the digestor in the event the pressure in the digestor drops below the pressure outside the digestor tank thereby avoiding vacuum which could collapse the tank.
The pressure relief valve used on a digestor tank must be operable at extremes of environmental temperatures including very cold temperatures. Additionally, the gas flowing through the valve may contain substantial amounts of moisture and some solid matter may be entrained in the gas. The solid matter can foul the valve and moisture in the gas can freeze on the valve surfaces in cold weather. Both of these conditions can hamper the operation of relief valves used on digestor tanks.